Flat panel displays are used in a wide variety of applications including, for example, televisions, notebook computers, projection systems, and wireless communications devices, such as cellular telephones. Images are formed on flat panel displays by electrically controlling the optical properties of a large number of individual picture elements, or "pixels," made of an electro-optical material, such as a liquid crystal material. The large number of pixels allows the formation of arbitrary information patterns in the form of text or graphic images by controlling the optical transmission of an arbitrary number of pixels. The optical state of each pixel, which depends upon the voltage present across it, is controlled by applying electrical signals to addressing electrodes. The number of electrodes necessary to address the large number of pixels is greatly reduced by having each electrode address multiple pixels. In a passive matrix display, transparent electrodes are typically positioned on opposing inner surfaces of parallel, transparent plates. A matrix of pixels is typically formed by electrodes arranged in horizontal rows on one plate and vertical columns on the other plate to provide a pixel wherever a row and column electrode overlap. Addressing signals determined by the image to be displayed in accordance with any number of addressing techniques are placed onto the electrodes by addressing signal voltage drivers. Multiple periodic addressing signals are required to display a complete image.
A complete image is typically displayed in a time interval known as a "frame period." To form an image during the frame period, rows are typically "selected," i.e., have a non-zero voltage applied, during "selection intervals" that comprise the frame period. Image-dependent column signals determined in accordance with the addressing technique are applied to the columns in each addressing interval. The optical response of the pixel is determined by the root mean square ("rms") of the potential difference over the frame period between the row and column electrodes.
Passive matrix liquid crystal displays typically use an Alt and Pleshko-type method of addressing the display, in which rows are selected sequentially during addressing intervals by the application of a row voltage, and, the column voltage applied during each addressing interval depends upon the desired optical state of the pixel defined by the row selected during the addressing interval and the corresponding column.
Image data indicating the desired optical state of the pixels during a frame period can be presented to the display in a variety of formats. Typically, the image data for the rows and for the pixels within each row are presented sequentially. Television signals present the pixel image data from all the odd numbered scan lines in a first "field" period and then the data from even numbered scan lines in a second "field" period. Control signals are typically interspersed within the image data. The term "addressing cycle" is used by applicants to mean either a field or a frame period.
Fast-responding liquid crystal displays are desirable because such displays are necessary for showing moving video images, which are produced by rapidly changing a series of still images. When a fast responding liquid crystal material is used, however, the liquid crystal material within a pixel has an opportunity to relax between successive selections of the row defining the pixel, causing an undesirable optical effect known as "frame response," described in Kaneko et al., "Full Color STN Video LCDs," Proceedings of Eurodisplay '90, pp. 100-103 (Tenth Annual International Display Research Conference, Amsterdam, the Netherlands, 1990).
A typical liquid crystal display may have 480 rows and 640 columns that intersect to form a matrix of 307,200 pixels. It is expected that matrix liquid crystal displays may soon comprise several million pixels. As the multiplex ratio, i.e., the number of matrix rows overlapping each column electrode, increases, the ratio of the time in which a row is selected to the frame period decreases. Each row, therefore, is selected for a relatively shorter time period, resulting in a decrease in the selection ratio, i.e., the ratio over a frame period of the rms voltage across an "ON" pixel to that of an "OFF" pixel.
A reduced selection ratio results in a reduced contrast ratio, i.e., the ratio of the light transmission of a light pixel to that of a dark pixel. A reduced selection ratio also results in a display having a slower response time, i.e., pixels are slower to change their optical state in response to changes in the addressing signals from frame to frame. A display having a reduced selection ratio also exhibits a narrower viewing angle.
One method used to reduce the multiplex ratio when addressing large numbers of rows, the "dual scan method," entails dividing a display into two separately addressed sections, with each display section having an independent set of column and row electrodes. The column electrodes of each display section overlap only the row electrodes of the same display section. A dual scan display typically has a higher contrast ratio than that of a "single scan" display having the same number of rows, but the dual scan display requires additional addressing hardware, including an additional set of column drivers. Moreover, because of the electrical connections required at the edges of the display panel, only two display panels can be vertically stacked without producing unacceptable gaps in the composite image.